The NOx in the air causes photochemical smog and acid rain. Therefore, exhaust of NOx from an automobile provided with an internal-combustion engine that is one of NOx generation sources becomes a social problem. Therefore, from now on, as to the exhaust amount of NOx, the investigation is advanced in the direction of severe laws and regulations. However, a diesel engine, and a lean burn gasoline engine are lean combustion in which oxygen is present in a large amount, therefore, NOx is hardly reduced and removed. Therefore, a NOx storage reduction catalyst for purifying exhaust gas, which is characterized in that NOx is stored in an oxidizing atmosphere, and the stored NOx is reduced and purified in a reducing atmosphere has been developed.
It is described that a cerium oxide is extremely useful for a NOx storage reduction catalyst for purifying exhaust gas, and NOx adsorption is performed by an oxygen storage-release function even in a low temperature range of 300° C. or less, and further, the cerium oxide itself also serves as a NOx adsorbent. (Patent Literature 1)
In addition, according to Patent Literature 2, it is described that the metal catalyst is heated by utilizing the reaction heat of cerium oxide having an oxygen storage-release function to enhance the catalytic activity.